Daily or more frequent samples of cervical mucus were collected from forty-two gilts and analysed for Na and K. In five gilts, sampled through several oestrous cycles, the K level exceeded that of Na for short periods during oestrus. The ovarian status of twenty animals indicated that the K peak occurred approximately 40 hr before ovulation but with some individual variations. Two of these animals did not show a K peak, and had never ovulated. The results differed in twelve gilts on a restricted feed level, the K level generally being in excess of Na. A K peak occurred in two immature gilts after injection of deoxycorticosterone.